This application is based on Japanese Patent Applications No. 2001-107097 filed on Apr. 5, 2001, and No. 2002-22348 filed on Jan. 30, 2002 the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an ignition system for igniting the air/fuel mixture in a combustion chamber. Especially, the ignition system has an ion current detecting circuit for detecting an ion-current.
2. Description of Related Art
JP-A-11-22615 discloses an ignition system that has an ion current detecting circuit. This system has a band pass filter (BPF) for rejecting a noise component and for detecting a high frequency component indicative of an occurrence of engine knock. A fundamental, first harmonic, component indicative of knock may appear in the same frequency range, but aflame noise also appears in the same frequency range. To reject flame noise, the prior system detects harmonic components higher than 10 kHz that include second harmonics of the knock signal.
Referring to FIG. 5, anion current when knock occurs is indicated by solid line 2, and ion current including a flame noise component is indicated by double dashed line 1. The first harmonic component indicative of knock is detected in the 5 to 7 kHz range, but the flame noise thereat has almost the same level. The second harmonic component indicative of knock is detected in the 10 to 14 kHz range, and here the flame noise is almost negligible. However, the ion current level varies in accordance with circuit arrangement or components of the ignition system.
For instance, the prior system has an ignition coil and a switching device that turns off primary current to induce secondary current in a secondary coil. This system needs relatively high inductance in order to induce sufficient energy on the secondary coil. But, on the other hand, the higher inductance reduces ion current level in a high frequency range. Therefore, it is difficult to achieve both sufficient ignition energy and high ion current sensitivity.
It is an object of the present invention to increase ion current level over a wide frequency range.
It is another object of the present invention to improve accuracy for detecting knock based on ion current.
According to a first aspect of the present invention, an ignition system with an ion current detecting circuit has an ignition circuit that has an energy accumulating coil, and an ignition coil. Therefore, it is possible to reduce inductance of a secondary coil in the ignition coil. The lower inductance secondary coil allows a greater ion current on a wider frequency range. Therefore it is possible to improve an ion current level. The ion current detecting circuit including an ion current detecting means, an extracting means and a detecting means can detect the knock accurately.
The ion current detecting circuit may extract a higher harmonic component. Since the ion current level on a higher frequency is increased, it is possible to detect the knock based on the higher harmonic component.
A current path where the current detected by the current detecting means flows may have a resonance frequency that substantially coincides with a frequency that is extracted by the extracting means. It is possible to improve the ion current level on the frequency to be extracted.
The secondary coil may have inductance less than 10 henries (H) to achieve a sufficient improvement.
The ignition circuit may have a control circuit which drives the first and second switching devices so as to provide a multi-spark ignition. It is possible to achieve a sufficient energy for an ignition sequence.